Out Of Bounds Write with Hmac Signatures

Out-of-bounds (OOB) writes in HMAC signature verification arise when an API's server-side code processes a maliciously crafted Authorization header or signature parameter without proper bounds checking, leading to memory corruption. This is distinct from generic buffer overflows because the attack surface is confined to the HMAC verification code path—a critical authentication gate.

Consider an API that uses HMAC for request signing. The server typically:

1. Extracts the signature from the header (e.g., X-Signature: hmac_sha256=...).
2. Recomputes the HMAC of the request body/headers using a secret key.
3. Compares the computed HMAC with the supplied signature in constant-time.

An OOB write occurs if step 1 or 3 mishandles the signature's length. For example, a C-based implementation might copy the signature into a fixed-size buffer:

char sig_buf[64]; // Expect 32-byte hex HMAC-SHA256
strcpy(sig_buf, supplied_signature); // Vulnerable!

If supplied_signature exceeds 63 bytes, strcpy writes past sig_buf, overwriting adjacent memory (e.g., return addresses, function pointers). An attacker could then hijack control flow to bypass authentication or execute arbitrary code.

Real-world CVEs illustrate this pattern. CVE-2022-1292 affected OpenSSL's EVP_PKEY_CTX_set1_id() where an oversized identifier could overflow a buffer during HMAC setup. While not exclusively HMAC verification, it shows how crypto libraries handling HMAC contexts are vulnerable. In APIs, the flaw often manifests in custom HMAC verification logic that:

• Uses unsafe string functions (strcpy, sprintf) on signature inputs.
• Assumes a fixed HMAC length (e.g., 32 bytes for SHA-256) without validating the actual input length.
• Parses hex/base64 signatures into buffers without size checks.

Attackers exploit this by sending requests with an extremely long X-Signature header (e.g., 10,000 'A' characters). If the server crashes or returns an authentication error that differs from a normal invalid signature, it may indicate a memory corruption vulnerability. More insidiously, a carefully crafted overflow could alter the comparison logic itself—making a forged signature appear valid.

Detecting OOB writes in HMAC signature verification requires fuzzing the signature parameter with abnormally long inputs while monitoring for crashes or behavioral anomalies. Manual testing involves sending progressively longer signatures and observing for:

• HTTP 500 errors (indicating a crash).
• Delayed responses (possible heap corruption mitigation).
• Authentication bypass (if overflow alters comparison logic).

However, automated scanning is more reliable. middleBrick includes an Input Validation check that specifically tests HMAC signature endpoints for buffer overflow risks. When you scan an API URL, middleBrick:

1. Identifies endpoints that use HMAC authentication (via OpenAPI spec analysis or runtime probing).
2. Submits requests with signature values ranging from 64 bytes to 10,000 bytes in length.
3. Analyzes response patterns: a sharp increase in 5xx errors or timing deviations suggests memory corruption.
4. Cross-references findings with the OpenAPI spec—if the X-Signature parameter lacks maxLength constraints, the risk score increases.

For example, a Flask endpoint using hmac.compare_digest safely in Python is resilient, but a legacy C module might crash under load. middleBrick's report will flag this under Input Validation with a severity based on exploitability. The CLI tool (middlebrick scan <url>) outputs JSON highlighting the vulnerable parameter and payload size that triggered the anomaly, which you can integrate into CI/CD via the GitHub Action to catch regressions.

Additionally, middleBrick's Authentication check reviews the OpenAPI spec for missing security schemes. If an API defines a custom HMAC scheme but doesn't enforce signature length limits in the spec, it's a design-time weakness that middleBrick surfaces as a prioritized finding.

Fixing OOB writes in HMAC verification requires strict input validation and safe memory handling. The remediation strategy depends on the language and library used.

1. Enforce Maximum Signature Length

Before any cryptographic operation, reject signatures exceeding the expected length. For HMAC-SHA256, the raw binary is 32 bytes; hex-encoded is 64; base64 is ~44 characters. Example in Node.js:

const MAX_SIG_LENGTH = 64; // hex-encoded SHA256
app.use((req, res, next) => {
  const sig = req.headers['x-signature'];
  if (sig && sig.length > MAX_SIG_LENGTH) {
    return res.status(400).json({ error: 'Invalid signature length' });
  }
  next();
});

This stops oversized inputs at the gateway, preventing them from reaching vulnerable HMAC logic.

2. Use Constant-Time Comparison with Bounded Inputs

Even with length checks, use constant-time comparison to avoid timing attacks. In C, avoid memcmp; use a safe function:

int safe_hmac_compare(const char *supplied, const char *expected, size_t len) {
  if (strlen(supplied) != len) return 0; // Reject wrong length
  return constant_time_compare(supplied, expected, len);
}

Libraries like OpenSSL provide CRYPTO_memcmp. Ensure len is the expected HMAC length (e.g., 32 for raw binary).

3. Avoid Manual Parsing When Possible

In higher-level languages, rely on built-in crypto libraries that handle decoding safely. In Python:

import hmac, hashlib, base64

def verify_hmac(secret: bytes, body: bytes, sig_header: str) -> bool:
    # Decode base64 signature, reject if malformed
    try:
        supplied_sig = base64.b64decode(sig_header, validate=True)
    except Exception:
        return False
    if len(supplied_sig) != hashlib.sha256().digest_size:
        return False
    computed = hmac.new(secret, body, hashlib.sha256).digest()
    return hmac.compare_digest(supplied_sig, computed)

The validate=True ensures only valid base64 is accepted, and the length check prevents OOB writes downstream.

4. Patch and Update Libraries

If using a vulnerable crypto library (e.g., an old OpenSSL version with CVE-2022-1292), update immediately. Monitor security advisories for HMAC-related flaws.

middleBrick's remediation guidance in the scan report will point to the specific parameter (e.g., X-Signature) and recommend length validation. For continuous enforcement, use the Pro plan's CI/CD gates to fail builds if new HMAC endpoints lack maxLength in their OpenAPI spec. The Dashboard tracks whether your HMAC implementations consistently meet secure coding patterns over time.

Out-of-bounds writes in HMAC signature verification are a high-risk vulnerability that can lead to complete authentication bypass. They stem from unsafe handling of the signature parameter—a single oversized header can corrupt memory. Detection requires active fuzzing of the signature field, which middleBrick automates as part of its Input Validation and Authentication checks. Remediation hinges on strict length validation, constant-time comparisons, and leveraging safe library functions. By integrating middleBrick's scans into your development workflow—via the CLI, GitHub Action, or Dashboard—you can systematically identify and eliminate these flaws before deployment.